Generally, it is desirable for monolithic circuits to be designed for minimum power consumption. The need for low power consumption is normally traded off with other factors such as integrated circuit cost and design time. However, recently integrated circuits are being used in a wide variety of products powered by batteries. In these products low power consumption is even more important because the higher the power consumption, the shorter the amount of time the products may be operated without replacing or recharging the battery.
Frequently, the circuits are in the form of microcontrollers. Microcontrollers are data processors having not only a central processing unit (CPU), but also other circuitry such as volatile and nonvolatile memory, timers, event counters, and input/output circuitry normally associated with computer systems. Microcontrollers may be affected by a drop in the power supply voltage. Not only will the microcontroller cease to function entirely when the power supply voltage falls too far, but they may also function incorrectly as the battery voltage falls, causing deleterious effects in the system.
In an effort to avoid this harmful operation, microcontroller designers have included circuitry to detect that the battery voltage has fallen to a critical level, and place the microcontroller into reset. These circuits are variously known as low voltage inhibit (LVI) or low voltage reset (LVR) circuits. Typically, the LVI circuits are analog circuits based on a bandgap voltage reference. A comparator detects when a fraction of the battery voltage drops below the bandgap voltage and takes some action in response, such as activating a reset signal to reset the microcontroller.
One difficulty faced by integrated circuit designers is that because the LVI circuit is an analog circuit, it consumes a significant amount of power itself, and it contributes to battery drain. Known techniques for reducing the LVI circuit's power consumption, furthermore, require an increase in integrated circuit area. What is needed, then is a low voltage inhibit circuit which is compact in terms of circuit area, and which has a minimum of current drain so as not to reduce battery life even further. Such a low voltage inhibit circuit and an integrated circuit using it are provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.